Transiting exoplanets: From planet statistics to their physicalnature

H. Rauer1,2

1Institut fur Planetenforschung, DLR, Rutherfordstr. 2, 12489 Berlin, Germany[heike.rauer@dlr.de]

2Zentrum fur Astronomie und Astrophysik, Technische Universitat Berlin,Hardenbergstr. 36, 10623 Berlin, Germany

Abstract. The colloquium ”Detection and Dynamics of Transiting Exoplanets”was held at the Observatoire de Haute-Provence and discussed the status of tran-siting exoplanet investigations in a 4.5 day meeting. Topics addressed ranged fromplanet detection, a discussion on planet composition and interior structure, atmo-spheres of hot-Jupiter planets, up to the effect of tides and the dynamical evolutionof planetary systems. Here, I give a summary of the recent developments of tran-siting planet detections and investigations discussed at this meeting.

1. The increasing planet statistics

It is fifteen years since the first announcement of an extrasolar planet (51 Peg b) or-biting a solar type star was made (Mayor et al. 1995), based on observations at theObservatoire de Haute-Provence (OHP). This colloquium, focusing on transiting extra-solar planets, follows a colloquium held at OHP in 2005 (Arnold, Bouchy & Moutou2006) which was organized to celebrate the anniversary of the detection of 51 Peg. Inthese last five years not only the detection statistics of exoplanets have increased sig-nificantly, but we have also seen the beginning of a new era of exoplanet investigations,addressing the physical properties of planets like their mass-radius relationships, atmo-spheric composition and orbital evolution. This development is a major step forwardtowards a meaningful comparison of extrasolar planets to the well-known planets inour Solar System and to a better understanding of planet formation and evolution ingeneral. The recent investigations of the nature of extrasolar planets were made pos-sible by the increasing number of known transiting extrasolar planets. Thus, althoughhampered by the low geometrical transit probability when considered purely from theviewpoint of detection statistics, the transit method has opened a new window for theinvestigation and understanding of extrasolar planets.

Figure 1 shows the current (Dec. 1, 2010) statistics of exoplanet detections (fromexoplanet.eu), indicating the different methods used. The fraction of known transit-ing planets has increased significantly since 2005. Most known transiting planets weredetected directly by the transit method, but transits are also observed for planets origi-nally detected via radial velocity (rv) measurements. In 2010, about half of the planetswere detected by rv and half by transit surveys. Several of these have been presentedat this conference. Ground-based transit surveys (e.g. WASP (Hellier et al. and Faedi

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Figure 1: Planet detection history. Blue: planets detected by radial velocity (rv) andwithout observed transits, red: planets with observed transits (detected by transit or rv),other: detected as indicated. (source: exoplanet.eu, Dec. 1, 2010)

et al., this volume) and HAT (Bakos et al., this volume)) presented 17 new planets.The Kepler team (Holman et al. 2010) discussed 2 transiting planets (Kepler 9 b andc) orbiting in the same planetary system. These planets show for the first time cleartransit timing variations. They also provided indications of a possible third planeton a short period orbit. Kepler-9 d was detected only after the transit timing varia-tion signal from the two planets already-discovered in this system was removed. It isstill difficult to rule out all false-positive contaminations, but recent work supports itsplanet nature even though its mass can not be derived yet (Torres et al. 2011). In 2010,the Kepler-mission released a first list of planet candidates which then triggered thedetection of further transiting planets and a white dwarf from rv follow-up of these can-didates (Hebrard et al., this volume; Ehrenreich et al. 2010). Furthermore, a planetarysystem with 7 planets was announced by the rv-team in Geneva (Lovis et al. 2010),including a small super-Earth planet of only 1.4 MEarth sin i. This non-exhaustive listof recent planet detections presented and discussed at this colloquium illustrates thesignificant increase in detection statistics made in the past few years. New surveys, likethe presented ASTEP400 project located at Antarctica (Crouzet et al., this volume),upgrades of existing surveys and the ongoing and future space missions will providemany further transiting planets for future investigations.

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2. Planets in the habitable zone?

The determination of the full planet mass function over all masses and orbital param-eters and for all types of central stars is key to understanding planet formation anddynamical evolution. In addition, the detection of small, terrestrial (super-)Earth plan-ets is the subject of special attention in all exoplanet surveys because these planets arepotential objects for habitability outside our Solar System. Figure 2 shows the locationof the habitable zone (HZ), defined as the orbital distance where liquid water can existon the surface of a terrestrial planet (Kasting et al. 1993). In our Solar System, onlyEarth is well within the HZ, whereas Venus and Mars are near its edges and have beenhabitable only for a short time interval in their evolution if at all.

Super-Earth (Transit + RV)

Super-Earth (RV)

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Figure 2: Illustration of the location of the currently known super-Earth planets withrespect to the habitable zone as defined by Kasting 1993.

Figure 2 also shows the location of the presently known super-Earth planets i.e.planets with m (sin i) < 10 Earth masses (parameters from exoplanet.eu). The majorityof super-Earth planets have been detected within 0.1 AU from their central star. Onlyone planet, detected by microlensing is found outside 1 AU, but since it orbits an Mdwarf star, it is well beyond the corresponding HZ. The super-Earth planets detected bytransits surveys up to now (CoRoT-7b, GJ1214 and Kepler-9d) are far too close to theirstar to be near the HZ. Today, only one exoplanet system (GJ581) has been detectedwith planets near the HZ. Recent model simulations (von Paris et al. 2010, Wordsworthet al. 2010) showed that GJ581d can indeed provide habitable conditions for certainCO2-rich atmospheres. Thus, one planet in the HZ is already known. Meanwhile,

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additional planets have been announced in the GJ581 system, including a planet wellwithin the HZ, GJ581g (Vogt et al. 2010). This planet would indeed show habitableconditions for a much wider range of atmospheric parameters. However, its detectionis presently controversial and needs to be confirmed (Pepe, F. 2010; Anglada-Escude2011).

Fig. 2 shows that the detection of super-Earth planets in the HZ is difficult upto now, for transits as well as for rv surveys. M dwarf central stars provide the bestchances due to the proximity of the HZ to the star in these systems. They, however,may have other difficulties if one considers the harsh radiation environment planetsmay suffer around these mostly very active stars.

In future, super-Earth detection surveys need to be improved to fill the gap betweenthe location of the HZ and the current detection range for terrestrial planets. In parallel,theoretical modeling activities are needed to better define the location of the HZ, e.g.for stars with very different activity levels than found in our Solar System.

3. Detection range of transit surveys

Figure 3 shows the transit depth (R2

p

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planet radii, Rp. The figure also shows the presently known transiting planets (blackdots). We can see that most detected transiting planets have radii around 1 - 1.3 RJ andorbit stars larger than about 0.7 Rs. The detected transit depth is between about 0.04%up to about 4%, with HAT-11b showing the smallest (so far) detected transit depthfrom ground-based observations which have resulted in a confirmed planet detectionso far. I arbitrarily chose HAT-11b therefore as the approximate empirical detectionlimit of current ground-based transit surveys (indicated as the green dashed horizontalline). This is not however a strict limit, and we may well soon see smaller detectedtransit depths as data reduction techniques and observing strategies improve - both forthe already-operating surveys, as well as for newly-planned future instruments. Here,it serves as a marker of what appears to be ”state-of-the-art” for ground-based transitdetection.

Since the transit depth increases for smaller central stars, it is possible to detectmuch smaller planets around M dwarf stars with the same detection thresholds. We cansee examples of such detections with the super-Earth planet GJ1214b and the super-Neptune GJ436b. The number of detections around M dwarf stars with R< 0.7 Rs is,however, still low because they have only recently started to become targets of dedicatedtransit search strategies (e.g. the MEARTH project (Nutzman & Charbonneau 2008)).Comparing the green solid lines, which indicate the transit depths of super-Earth andEarth-sized planets with the current empirical detection limit of ground-based surveys,we can see that terrestrial planets may be detected from the ground but currentlyonly for those planets orbiting the smallest M-dwarfs. Of course, surveys of M dwarfstars also have to overcome the additional difficulty of the much fainter target stars.Nevertheless, this parameter range is highly interesting for future transit searches,because of the close location of the HZ around cool M dwarfs (see Fig. 2).

The blue-dashed horizontal line in Fig. 3 marks the observation of transits ofHD149026b. This planet was previously detected by rv-measurements, and the follow-up transit observations were then made with a larger aperture telescope than usuallyused for transit detection surveys. I have therefore arbitrarily indicated the regionbelow the blue-dashed line as the domain of space-based surveys. Terrestrial planetsaround solar-type stars can be detected only from space and seem out of reach forground-based surveys. The smallest planets in this regime are CoRoT-7b (Leger et al.

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CoRoTCoRoT 7b7b

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Figure 3: Transit depth versus stellar radius for different planet radii. Black dots showposition of known transiting planets.

2009, Queloz et al. 2009) and Kepler-9d (Holman et al. 2010). Both also illustrate thedifficulties to confirm such low-mass planets and determine their mass. As discussedbelow, the mass of CoRoT-7b is still somewhat uncertain, and the mass of Kepler-9d(needed to confirm its detection) has not yet been determined.

The current space missions CoRoT and Kepler as well as future missions proposedin Europe (PLATO) and the U.S. (e.g. TESS, ASTRO) target the detection of smalltransit depths to fill the known planet mass function at its low mass and radius endand to detect terrestrial planets with known radii and masses for future spectroscopiccharacterization. For small M dwarfs sensitive space surveys could even detect planetswell below Earth size.

4. Planet basic parameters and spectral characterization

Planets with observed transits provide a huge potential for the detailed investigation ofthe objects found. The most surprising discovery in the last few years was made via theRossiter-McLaughlin (RM) effect. It allows us to determine the angle between the pro-jected rotation axis of the planetary orbit and the stellar spin axis as well as the sense ofplanet revolution by precise rv-measurements during planet transits. Surprisingly, outof about 30 planets with detected RM effect (Nov. 2010) approximately 30 % showedhighly inclined orbits, and some even show evidence for (potentially) retrograde orbits.These findings have stimulated numerous modeling efforts to explain the dynamicalevolution of such planets, and were also a focus of this colloquium. In particular theneed for improved modeling taking into account the stellar-planet tidal interaction was

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identified and mechanisms of tidal interaction (e.g. the Kozai effect, tidal inflation,etc.) were reviewed (Mardling, this volume). Cebron et al. (this volume) discussed thepotential role of elliptical instabilities that might be able to affect the stellar rotationaxis, thereby providing another mechanism that could explain the observed inclinedsystems.

The combination of radii derived from transits and masses from rv-measurementsallows us to place a planet in modeled mass-radius diagrams and to determine its meandensity. While this is possible with good accuracy for Jupiter and Neptune-sized plan-ets, it is however still challenging for terrestrial objects. An example discussed at thisconference, illustrating the state-of-the-art in determining the basic planet parametersof terrestrial planets, is CoRoT-7b. Its derived radius and mass was updated severaltimes since the detection announcement (Leger et al. 2009, Queloz et al. 2009) due torevised stellar parameters (Bruntt et al. 2010) and the potential presence of a thirdplanet in the rv-data (Hatzes et al. 2010). At this meeting, the additional effect of stel-lar activity in the radial velocity data analysis was discussed (Pont, Aigrain & Zucker2010). A recent independent re-analysis of the rv-data increased the mass range forCoRoT-7b again (Ferraz-Mello et al. 2010, Boisse et al. 2010). While the planetradius derived from the transit measurement now seems well constrained subsequentto revising the stellar parameters, the published range of planet masses range from asmall terrestrial planet up to a large super-Earth. This ongoing discussion shows thatterrestrial planet detections around a relatively faint star such as CoRoT-7b (V=11.7mag) are near our current limit of observations and data reduction. The discussion,however, gives clear guidelines for future transit and rv-surveys, namely to focus onmuch brighter target stars where such controversies can be solved easier. In addition,the rv follow-up strategies have to be optimized for such controversial cases, for exampleby simultaneous rv- and photometric measurements.

Once radius and mass are well known, they can be used to constrain the interiorstructure of planets. For hot-Jupiter planets the detected radii span a wide parameterrange for similar masses. For the large, inflated planets, this observation still remains tobe fully explained by theory. The investigation of mass-radius relationships of terrestrialplanets has just begun (e.g. Valencia et al. 2010, Wagner et al. 2010). Whether asimilar spread in basic planet parameters (radius and mass) is also found still needs tobe determined by increasing the number of known transiting terrestrial planets.

Transiting planets furthermore provide the opportunity for spectroscopic charac-terization of their atmospheres. Observations during primary and secondary eclipsesprovide data on the planetary atmosphere. Orbital phase variations due to reflectedlight have also been detected. Such characterizing observations are usually made at lowresolution. Today, they are restricted to hot-Jupiter and Neptune sized planets aroundthe brightest host stars. Future space telescopes aim to reduce this limit to includesuper-Earth planets in such spectroscopic studies.

5. Summary

In summary, this colloquium has shown that scientific investigations of extrasolar plan-ets have entered a new phase in recent years, from pure detection statistics to investiga-tions of the physical nature of planets. This development is expected to continue overthe years to come with the help of new observing facilities from ground and space, e.g.the ELT, JWST, proposed future transit missions, like PLATO, TESS, ASTRO, butalso planets detected by other methods (e.g. direct detection) which will give access toan expanded parameter range.

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